Michelle Carter acted with criminal intent when she “badgered” Conrad Roy III into killing himself in 2014, the state’s high court ruled Wednesday as it upheld her involuntary manslaughter conviction and 15-month prison sentence.

In a unanimous ruling, the Supreme Judicial Court rejected legal arguments that Carter’s text messages and cellphone calls with Roy were forms of free speech protected by the First Amendment, and instead applied a 203-year-old ruling to her 21st-century communications.

“We conclude that the evidence was sufficient to support the judge’s finding of proof beyond a reasonable doubt that the defendant committed involuntary manslaughter as a youthful offender, and that the other legal issues presented by the defendant, including her First Amendment claim, lack merit,’’ Justice Scott L. Kafker wrote for the court. “We therefore affirm.”

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Carter was 17 years old and had been out of a psychiatric hospital for about a month when she urged her boyfriend, Roy, 18, to commit suicide on July 13, 2014, according to testimony in her Bristol Juvenile Court trial in 2017.

Carter was 30 miles away from Roy and on the phone with him, listening as he inhaled carbon monoxide in his pickup truck in a Fairhaven parking lot, according to testimony. At one point, Roy told Carter he was getting out of the truck, but Carter ordered him back in, prosecutors said.

The Plainville woman was convicted of involuntary manslaughter in a jury-waived trial in 2017 and sentenced by Juvenile Court Judge Lawrence Moniz to serve 15 months in prison. The sentence has been on hold while the high court reviewed her trial and the defense argument that her actions were not criminal in nature.

Daniel N. Marx, an attorney for Carter, said in a statement that her legal team was “disappointed in the Court’s decision.”

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Marx said the ruling “adopts a narrative that we do not believe the evidence supports. We continue to believe that Michelle Carter did not cause Conrad Roy’s tragic death, and she should not be held criminally responsible for his choice to end his own life. Today’s decision stretches the law to assign blame for a tragedy that was not a crime.”

The decision, Marx said, “has very troubling implications, for free speech, due process, and the exercise of prosecutorial discretion, that should concern us all. There are good reasons why nearly every other state has passed a law to address ‘assisted suicide,’ which inevitably involves complicated circumstances better addressed as a matter of policy by the legislature than in any particular case by the court. We will evaluate all legal options for Michelle including a possible appeal to the US Supreme Court.”

Bristol District Attorney Thomas Quinn III said in a statement his office will ask the Juvenile Court in the near future to order Carter to begin serving her prison sentence. Quinn said he was “pleased” that the SJC upheld Carter’s conviction.

“This case is a tragedy for all of the people impacted by this case,’’ Quinn said in the statement. “Her conduct was wanton and reckless, and caused the death of Conrad Roy. This type of conduct has long been a crime in Massachusetts.”

“I know it was a tough decision,’’ she said of the prosecution of Carter for her grandson’s death. “But we are very happy. . . . I am glad that justice will be served.”

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Janice Roy said she attended every hearing during the trial phase and will be in court when Carter, as expected, is ordered to start serving her sentence.

“It will be a good end, a kind of closure,” she said.

In the 33-page decision, the high court referred to its earlier ruling in the Carter case, when it concluded that there was enough evidence for the matter to go to trial — even though Carter was not in direct physical contact at the time of Roy’s suicide. In the aftermath of her conviction by a judge, the court said Wednesday that it was once again making it clear Carter was properly tried.

Roy, Kafker wrote, was a “vulnerable, confused, mentally ill, eighteen year old” who had gotten out of his pickup truck filled with carbon monoxide fumes into fresh air.

“But then in this weakened state he was badgered back into the gas-infused truck by the defendant, his girlfriend,’’ Kafker wrote. “After she convinced him to get back into the carbon monoxide filled truck, she did absolutely nothing to help him: she did not call for help or tell him to get out of the truck as she listened to him choke and die.”

Roy and Carter called one another boyfriend and girlfriend but had only met a couple of times in person.

Kafker added: “There is no doubt in this case that the defendant wantonly or recklessly instructed the victim to kill himself, and that her instructions caused his death.”

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The court responded to arguments from the American Civil Liberties Union of Massachusetts and others who saw the criminal case as an infringement of Carter’s First Amendment rights because her actions involved only words, not any physical act.

Massachusetts’ involuntary manslaughter law covers “wanton and reckless conduct” that causes the death of another and that includes “overpowering [another] person’s will to live and resulting in a person’s death,’’ Kafker wrote.

The court also cited an 1816 case in which a man was prosecuted for urging a man in an adjoining prison cell to commit suicide. The principle was applied in a 1961 case, the court said.

“Our common law provides sufficient notice that a person might be charged with involuntary manslaughter for reckless or wanton conduct, including verbal conduct, causing a victim to commit suicide,’’ the court ruled.

One concern raised by lawyers was that Carter’s case would lead to prosecution of loved ones as they discuss end-of-life issues, which could include a discussion about suicide.

“This case does not involve the prosecution of end-of-life discussions between a doctor, family member, or friend and a mature, terminally ill adult confronting the difficult personal choices that must be made when faced with the certain physical and mental suffering brought upon by impending death,’’ the court wrote. “Nor does it involve prosecutions of general discussions about euthanasia or suicide targeting the ideas themselves.”

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But Matt Segal, legal director of the ACLU of Massachusetts, which filed an amicus brief on Carter’s behalf during her appeal, said the ruling gives the government overly broad powers.

“Conrad Roy’s suicide is indisputably tragic,” Segal said in a statement. “But, by upholding Michelle Carter’s conviction, the Supreme Judicial Court has handed prosecutors broad, undefined powers that diminish the speech rights of everyone in Massachusetts. The Court’s decision tells prosecutors that they can bring charges against individuals based on arbitrary and subjective determinations of what speech is noble and what speech is criminal.”

Segal added that such “prosecutorial power will chill important and loving end-of-life discussions between family members, and could lead to erroneous convictions of children who engage in reckless, juvenile conversations with friends. The Court’s assurance that its opinion will not broadly criminalize other speech does little to change this inevitable outcome.”